CA1121285A

CA1121285A - Filter elements for gas or liquid and methods of making such filters

Info

Publication number: CA1121285A
Application number: CA000319636A
Authority: CA
Inventors: Brian Walker; Kenneth Merrie
Original assignee: Process Scientific Innovations Ltd
Current assignee: Process Scientific Innovations Ltd
Priority date: 1978-01-23
Filing date: 1979-01-15
Publication date: 1982-04-06
Also published as: JPH0230286B2; DE2902347A1; AU523755B2; SE7900495L; ATA40179A; AU4356279A; NL7900508A; DE2902347C2; SE436543B; US4303472A; DK153291B; BR7900397A; DK153291C; GB1603519A; BE873561R; US4272318A; US4360433A; AT376903B; CH638689A5; JPS54108974A

Abstract

ABSTRACT OF THE DISCLOSURE
Tubular filter elements are formed by first feeding a dispersion of fibres in liquid into a tubular moulding space between a vertical core and a cylindrical fine mesh screen.
Air pressure is applied to the ispersion so that the liquid drains through the screen and through a screen at one base of the space, while a mass of microfibres builds up to be removed from the space for bonding by a synthetic resin. A
reciprocable sleeve increases the effective height of the screen as the mass builds up. The process packs the fibres in the circumferential direction more uniformly than is possible with prior techniques providing high filtering efficiency and a capacity to withstand high differential pressures.

Description

Z~

This invention relates to filter elements and to methods for making filter elements.
In Canadian Patent 1,077,864 issued May 20, 1980, entitled "Filter Elements for Gas or Liquid and Methods for Making Such Elements" and assigned to the present applicant, there is described a method of forming a filter element which comprises dispersing a mass of fibres in a liquid to form a slurry, draining the liquid through a filter surface on which the fibres collect while an apertured sheet of supporting mat-erial is located at a selected distance above the filter sur-face, so that the fibres build up from the filter surface through the apertures in the supporting material to a predeter-mined distance above the supporting material, removing the col-lected fibres containing the sheet of supporting material from the filter surface, and bonding the fibres to one another and to the supporting material by means of a synthetic resin.
One aspect of the present invention is based on further experiments that demonstrate unexpectedly satisfactory results if, in the aforesaid method, the sheet of supporting material is omitted or, if provided, is located substantially in contact with the filter surface so that the sheet of supporting material ~ i becomes moulded into one surface of the filter element. The invention results in the production of a particularly efficient fibrous filter that is very economical to manufacture.
According to the present invention, a method forming ~` a filter element comprises dispersing a mass of fibres in a liquid to form a slurry, applying the dispersion under pres-sure to a filter surface so that the fibres collect as a layer covering the filter surface while the liquid passes through the filter surface, and bonding the fibres in the `~ collected mass of fibres, after drying to one another by ~ means of a synthetic resin. A sheet of material that is to :` ~
: .

Z~5 ..
provide a support for the filter element may be mounted in contact with at least a portion of the filter surface so that the support sheet becomes mouled into one surface of the col-lected mass of fibres. Thus, when the support shee-t, which may be provided by very fine mesh material, is removed, the fibres are found to have penetrated through the support sheet leaving their outer surface flush with the outer surface of the support sheet, which may be a layer of expanded metal. In the past, in the case of cylindrical filter elements, these have requirec' the addition of a separate support sheet to give strength, but the present moulding method enables the filter cylinder and support sheet to be produced as an integral part `~
in one operation with precision, saving time and labour.
More specifically the present invention relates to a method of forming a filter element comprising forming a dis-persion of fibres in liquid, introducing the dispersion into a container having a base wall and at least one side wall con- --stituted by a filter screen which extends from the base wall and whose effective pervious area is arranged to be increased by ;
20 an imperforate screen as the imperforate screen is withdrawn ~.
from the base wall, applying positive pressure to the dispersion being introduced into the container to compact an accumulating mass of the fibres on the base wall while aiding the discharge of the liquid through the filter screen and progressively in- :
creasing the separation between the imperforate screen and the base wall so as to increase the effective pervious area of the filter screen at a rate substantially equal to the rate at which ` the accumulating mass of fibres extends from the base wall, ; and removing the mass of fibres after they have accumulated to 30 a desired amount either alone or with the side wall moulded -into the surface of the mass.

The present invention also specifically relates to a

-2- ;~ :

~ . , self-supporting fibrous filter element comprising a tubular wall consisting of a mass of fibres compacted at substantially constant density throughout the length and thickness of the tubular wall with a majority of the fibres disposed substan-tially parallel to one another in a circumferential direction about the central axis of the tubular wall and layers at sub-stantially constant density substantially perpendicularly to the axis, and a synthetic resin bonding the fibres together.
In general, it is desirable to make the apertures and open area of the support sheet as large as possible. However, it is difficult to specify the largest aperture that can be used. The smallest aperture at present contemplated is 0.25 mm diameter. However, it must be remembered that certain fibres, such as potassium polytitanate, e.g., potassium dititanate, have a diameter of 0.5 microns and length of up to 0.15 mm and these can penetrate apertures of 0.25 mm and smaller.
In the case of the filter surface, expanded metal with narrow flat strips between overlapping elongated aper-tures, an aperture si~e of 0.755 mm by 0.5 mm has been found to be practical. This gives a good surface finish. 1 mm by 0.75 mm will of course give a somewhat rougher finish.

$ -3 -. ~ , . . , , . ~

S

Other practical examples or rigid s~pports have had apertures of 2.8 mm by 0.8 mm providin~l an open area of 26%
of the area of the support sheet, and 43 mm by 2C mm ~ith an open area of 83~. In general it has been found that the support sheet results in a very small flow restriction, of : the order of l~ ~o 2~ of the total flow.
In a modific3tlon of the aforesaid ~ethod, the support sheet consists of d rigid foam or sintered material tnereby eliminatin~ the necessity for the use of the very fine mesh material in the production of the filter element.
When, as in the aforesaid prior s~ecification, the binder is iised not only to bond the fibres together but also to the support sheet, this may be, for example, silicone, ` polyurethane, expoxy or phenolic resin. Heat cured resins are preEerred though air drying resins can be used. The weight of the resin binder depends on the strength required.
Generally the weight of the binder is no more than lOO~ of the weight of the fibres.
It has been found that the use of pressure in the method according to the invention results in a majority of the fibres being disposed so that they are directed, in some measure, approximately in parallel with one another. This gives particularly advanta~eous results, whether or not a support sheet is used. According to a further aspect of the invention, therefore, a filter element comprises a mass of - fibres compacted together and bonded to one another with a synthetic resin, a majority of the fibres being disposed so that they are directed, in some ~easure, approxiinately in parallel with one another.
In orcier that the invention may be clearly understoocd .

and readil~- carried in~o effert, e~amples of the invention ~
no-~ he described ~ith reference to the accom~anyina dra~in~s, in Yhich:
Figure 1 is a sec~ional elevation of part of a filter element;
Fi~ure 2 is an enlar~ement of a portion of Figure 1;
Figure 3 is a see~ional elevation of part of anothe filter element;
- Figure 4 is a dia~ram shol~ing apparatus for manltfacturin~
a filter element;
Figure 5 is a sectional elevation of a detail of the apparatus of Figure 4;
Figllre 6 is similar to Figure 5 b~lt relates to a different phase in the operation of the apparatus;
Figure 7 is a sect~onal elevation of a further filter element; and Figures 8 to 18 show portions of ~arious sealing arrange-ments for the ends of filter elements.
The portion of the filter element shown in Figures 1 and 2 may be part of the wall of a cylindrical filter element although ~i it can equally well be regarded as part of a disc, sheet or conical or frusto conical cylindrical shape (for example closed at one end as sholm in Figure 7). A similar method may also be used for the production o~ concave or convex discs. The bulk 1 2$ of the filter element comprises fibre material; for example 9 ~ glass, cera~ic, synthetic fibres, asbestos, mineral wool~ organic i or silicate fibres. R~w borosilicate microfibre is a preferred ~aterial. For cartridge type filters to be used in liquid filtration~ cellulose, ~Jool~ synthetic ~olymer (e.g~ polypropy-lene and acrylic~ fibres~ and combinations of these~ also such combinations containing a portion of borosilicate microfibre can very advantageously ~e ~sed. These combinations can also .. .. .

S

to be used for gas filtration. Both faces of the fibre mass 1 have an apertured suppor-t sheet 2 moulded thereto so that the fibrous mass penetrates through the apertures in the sheets to present surfaces that are flush with the outer surfaces of the sheets (Figure 2). Each support sheet consists of an apertured or open pore rigid material such as a perforated, expanded or woven material which, in turn, may be of metal, plastics, glass or ceramic. Expanded metal is a preferred material.
The filter element of Figure 3 is similar to that of Figure 2, but only one support sheet 2 is used. Where one support sheet is used, this is generally located on the downstream side of the fibres. This not only gives strength where it is required but does not reduce the inlet surface area of the filter, thereby increasing the dirt holding capacity. For low pressure use as for example in vacuum systems, the support sheet can be of comparatively light construction but, when used in a high pressure system, either with gas or liquid, the support sheet can be of heavier construction. ;
In a further example consis-ting of a cylindrical filter element, no support sheet is used. This example consists of a tube made from raw borosilicate microfibre moulded by pressure forming into the cylindrical shape by a method as described below with references to Figures 4 to 6.
The moulded tube is then dipped into a solution of resin in a solvent so as to impregnate the fibrous material and is then heat cured. By using a method as described below a filter ~` element without any support sheet can be constructed with very advantageous properties. For example such filter elements 54 mm long, 44 mm outside ~ -6-.
.

ZBS

diameter and 34 mm inside diameter have been constructed and tested to ~ive the following characteristics:

D.O.P. BURST ~LOIi ~p p O. D . T .
~ bar N~3/~ bar bar ~ w/~
. . .

99.999 >7.0 45 .069 ?.0 15.0 99.97 >7.0 45 .069 3.0 27.0 9~.90 >7.0 ~8 .035 3.0 25.0 10 99.80 >7.0 52 .035 3.0 21.0 99.80 >7.0 50 .035 4.0 35~0 In the above table p is the operatin~ test pressure, ~ is the pressure loss across the filter below and O.D.T. is the ratio of the oven dried total weight of resin to the fibre content of the filter element. The binder used in all the filter elements represented in the above table was a silicone resin, which is preferred, but many other binders can be used to give comparative test results. The highest resin content which is in the last tabulated example, is 35~
but this can be raised as hi~h as 100% while still providing satisfactory characteristics. ~lowever~ 25% has been found admirably satisfactory for most applications.
The effect on performance of varying the wall thickness of an unsupported tubular filter element is shown in the following table relating to a larger element 200 mm long, 56 mm outside diameter and 54 mm inside diameter for sample (a) but 46 mm inside diameter for example tb):-'.

SAMPLE D.O.P. FLOW ~P P O. D . T.

% NM /H bar bar % w/w (a) 99 99 306 .017 4.2 16.0(b) 99.999 170 .017 4.2 16.0 In the above table the pressures p is a gauge pressure above atmospheric pressure while ~p, of course, is a pressure differential.

The above table shows that it is effectively only the Elow capacity and efficiency that is affected by the increase in wall thickness. In practice, it is thought that about 3 mm will prove to be a lower limit for the ~all ~ thi~kness.
-` The good results, exemplified by the above tables, -are believed to arise from the packing pattern of the fibres `` .
that arise as a result of a method of manufacture such as described below with reference to Figures 4 to 6. This . ~
packing pattern results from the fibres lying in some ~o measure more uniformly in a circumferential direction round the filter elementr than is possible with known vacuum ~; methods which display a totally random packing pattern.
The more regular packing in the filter elements of the invention does not detract from their efficiency.
Although the filter element described immediately above have no rigid support sheet, they can be provided with an inner, outer, or both inner and outer layer of woven or non-woven flexible material to improve the handling characteristics. Such a layer can be incorporated during the manufacture of the filter element by a method as ; described below. The fibres would generally penetrate ' ~

: . , through an aperture or poxe structure of the flexible material. Moreover in the case of a filter element with a single rigid support sheet as shown in Figure 3, the opposite face of the fibrous structure can be provided with a layer of flexible material.
Simple, unsupported tubular filter elements as ; described above may be formed with a variety of surface patterns for example circumferential or longitudinal grooves, to increase the surface area.
Figure 4 shows diagrammatically apparatus for forming ~` a tubular filter element. When this apparatus is in operation, water and borosilicate microfibres are fed into a blending tank 31. Hydrochloric or sulphuric acid is added until the pH value reaches 2.8 to 3.5. Borosilicate microfibres are found to disperse more readily at this value. It has also been found that the fibres disperse more readily if the solution temperature is increased to about 35C. The quality of the fibres that are used depends on the grade of the filter element that is to be used. The fibre to water ratio (by weight) is generally 0.05% but can vary between 0.01~ and 0.5%. A binder such as colloidal silica may be introduced into the slurry at this stage. It has been found advantageous to use this type of binder to ,~ . .
impart additional strength prior to resin impregnation. The final dispersion is effected by a mechanical agitator 32 and takes about 15 minutes.
With valves 33 and 34 closed and valve 35 open, a pump 36 transfers the dispersion to a pressure vessel 37. The precise quantity transferred depends on the fibre/water ratio and the size of the filter element to be produced.

~,.

.. . .. ... .

2~Zc~S

Ne~t the valve 35 is closed and the valve 33 is o~ened to admit compressed air to the pressure vessel 37.
Generally the peessure used is 3.5 bar. This top pressure is the formina pressure and can be varied according to the efficiency required. The efficiency can be varied within a ran~e, e.g.~ 99~9~ to 99.999%, using the sa~e fibre blend.
The formin~ pressu~e may be as low as 0.3 bar, but a pressure of 3.5 bar has been found highly satisEact~ry with the fibre blend adjusted to suit the required efficiency.
The next ste~ is to open the valve 34 to enable the dis~ersion to flow into a moulding rig 38 shown in detail in Fi~ures 5 and 6. The moulding rig includes inner and ~ outer vertical cylinders 39, 40 definin~ a space 41 through ; which the dispersion can flow into a cylindrical moulding lS space 42 defined between a ~ine mesh screen 44, supported by a machined perforated cylinder 45~ and a core 43 when in the position of ~igure 5. Figures 4 and 5 show the filter element being moulded as a unit with an outer rigid cylindrical support sheet 2, but lt will be appreciated that for a si~ple borosilicate microfibre filter tube, this can be omitted. Alternatively, of course, an inner support sheet can be moulded into the inside surface of the tube, either instead of or as an addition to the outer sheet 2.
The bottom of the mouldin~ space is covered by a fine mesh screen 46. A reciprocable sleeve 47 is mounted to slide outside the cylinder 40 and perforated cylinder 45 With the core 43 and sleeT~e 47 in the positions shown in Figure 5, the water drains away through the screen 46 and lower end cf the screen 44 into a tank 48 (Eigure 4) while the mass of fibres begin to builcl up in the moulding , . . .

space 42. After all the fibres have accummulated in the moulding space, the air pressure is maintained so as to remove residual water from the fibres and so dry the formed `filter. The valve 34 is then closed. During the moulding process, a pump 49 continuously pumps the water fro~ the tank 48 to a holding tank 50 from which the water is recycled.
Finally the core 43 is removed to enable the formed filter element to be removed from the rig 38. The process c` 10 can then be started once more. As an example, it has been found that the time taken to mould a tubular filter element 250 mm long, 65 mm outside diameter with a wall thickness of 10 mm takes approximately one minute. The formed filter element is removed to a hot air dryer for final drying and is then resin impregnated and oven cured to harden the resin.
Particularly in the case of long filter elements, e.g., over 50 mm, it has been found desirable progressively to raise the sleeve 47, substantially at the same rate that `20 the height of the fibre mass increases, in order to maintain an uninterrupted flow of the dispersion to a point where the mass of fibres is building up. The movement of the sleeve 47 then terminates as shown in Figure 6.
The core 43 is formed with an upper portion 51 of reduced diameter. This is to enable an additional internal layer of fibrous filter material to be added to the filter material formed in the moulding space 42, by feeding a further dispersion through the cylinder 39 into a moulding space 52 (Figure 6) between the moulding space 42 and the core portion 51 when the core 43 is lowered. The water ~' , . .
, ~ .

z~
from the ne;~ la~er escapes throu~h the fibres in the space 42. The new layer may he O-r higher or lower efficiency than the tubular element formed in the space ~n~. This ~rrangement enables a ,filter eler,~ent of graded density to be produced as part of an inte~ral process.
Investigations have shown that the fibres in a finished filter element produced by the method described ab~ e with ~ reference to Figures ~ to 6 are predomi~antly layered in - planes perpendicular to the direction in which the dispersion flows into the ~oulding space. It has further ~een ~ound that the same packing pattern arises throughout the range of forming pressures that can be used effectively in practice.
Advflntages of this packing pattern appear from the results tabulated above.
i5 For some applications of the inYentiOn~ where cellulose , fibres or combinations of cellulose fibres with borosilicate fibres are usedt a melamine or phenolic resin binder may advantageously be used for the bonding material. Cellulose when bonded with melamine resin is approved as being suitable lor potable water and sanitary conditions. Phenolic resin is preferred for higher temperature workO The combination of cellulose fibres with other fibres provides eco~omies both in regard to cost and production time, good flow characteristics and chemical resistance, and controlled selection of pore ~5 size by blending different fibre materials with cellulose.
It has been found that by blending 20% borosilicate microfibre with 80~ cellulose by 7~eight the production time fnr the filter can be reduced by 30%. In this case when the fluid is water the pressure drop (~p) across the filter was 0~15 bar with a flow rate of 16 litres per minute. With a weight to weight ratio of 50~0, ~p was found to be O.i5 bar with a flow rate of 22 litres per minute. The glass fibre size (diameter) ~ . -- ~ _ ~ . ~ _ __ _ _ ~ , . _ _, _. _.. _ . _ . __ _ , ...... , _ . _. _~ . _ _ . _.. ~. . _ ~, . ... ..
. _.. .

s was 3.8 to 5.i microns and the cellulose ~ bleach~ sort~ood kraft. The bonding materictl, e.g. melamine resin, phenolic resin or other synthetic resin9 c~n be applied in one of three di-rferent ~ays~ Firstly, by forming a mass of fibres in a moulding rig such as sho~m in Figures 5 and 6, then t~e impregnating/mass after drying by dipping in a resin solution and curing the resin in an oven. Secondly, by preparing the cellulose fibre and separately mixing the borosilicate fibre with a resin solution, bringing the two mi~tllres together, ~-orming the mass under pressure in the moulding rig and curing the mass. Thirdly, all the fibres and resin solution can be mixed in a single tank, passed to the moulding rig, the mass being subsequently cured.
A cylindrical filter element for liquid filtration having a combination of fibres as described above may have an outside diameter of 64 mm, a wall thickness of 18 mm and various lengths, such as 250 mm. No support sheet is necessary for many uses but can be added when necessary.
The filter is preferably arranged for flow from outside to inside the oylinder to give greater surface area for collection of dirt. This area can be increased by forming longitudinal or circumferential grooves in the outside surface of the cylinder.
Instead of using a compressed gas to apply pressure ~5 to the slurry in the moulding rig, a hydraulic pump may be used, this pump being arranged to withdraw the slurry from the blending tank and ~orce it into the moulding rig, : ,. , , : , :
Tubular or cylindrical ~ilter elc-~ents ~ade in accord~nce with the in~ention may be mounted in a variety of fIlters, in particular those shown in Fi~lures 5, 6, 7 and 13 in the aEoresaid specification. As in that s~ecification, also the ends of the cylindrical filter elements may be fitted into-end caps in a variety of ways.
Such ways are shown in Fi~ures 8 to 17 of t~e present specific~tion.
F~igures 8 to 13 show cases wnere tlle end of a cylindrical, unsu~ported filter element 10 is ~itted into an end cap 11 using a gasket seal 12 (Figure 8), a double taper seal 13 (Fi~ure 9), an outside taper seal 14 (Figure 10) an inside t~per seal 15 (Figures 11 and 12) and a double ta~er flanqe seal 16 (~i~ure 13). For a cylin~3rical filter element with an inside support sheet 17 an ou~side taper seal 14 (Fi~ure 14) may be used. For an oatside support sheet 18 (Figure 15) an inside taper seal 15, or a singlè
taper flange seal 19 (Fi~ure 16) may be used. In the case of a filter ele~ent having insicle and outside support sheets 20, 21 (Figure 17) a gasket seal 12 (Fi~ure 17) can be used.
In all forms of the filter ele~ent constructed accordin~ to the invention, an open pore filter layer of sleeve, as shown in Figures 12 and 17, can be used if required to act as a pre-filter or as an after-filter to drain coalesced liquids.
This layer or sleeve can be an open pore plastic or metal foaln or a layer or layers oE non-woven material such as felt. As a further alternative the Eilter element can be dip sealed into end caps as shown in` Fi~ure 6 of the aforesaid specification. Figure 18 shows an arrangement similar to Fi~ure 15 with an internal supporting spring 24 in place of any inside support sheet.

- ~4 -.:

Filters made in accordance with tlle invention can be used for either gas or liauid filtration. The efficiency can be as high as 9~.9999~ s~hen tested to BS 9~00 or can be produced with a micron rating in various st39es between 1 and 50 microns. A further method of increasing the efficiency of the moulde~ filter material is by comoressiny the material while being resin impreanated and cure~.
A further material that can be used for the support sheet is a rigid metal foam. The fibr2s can be moulded directly onto sucn foam so that they penetrate only so far into the thiclcness of the foam sheet, but the fine mesh screen 44 can be eliminated in this process because the foam sheet itself provides the filter surface throu~h which the water is drained. The same method can be used in the case f the aforesaid sintered sup~ort sheet. ~he same metho~
can also be used with foam consisting of plastics material, which may be flexible or semi-rigid. Elowever, very advantageously a rigid polyvinyl chloride coated plastic foam can be used.
~0 Among the many possible uses of the filter accordin~
to the inven.ion are the removal of oil from compressed air, pre-filtration, aeration~vacuum filtration, liquid filtration, air sterilisation and for pneumatic silencing.

- 15 ~

,

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of forming a filter element comprising forming a dispersion of fibres in liquid, introducing the disper-sion into a container having a base wall and at least one side wall constituted by a filter screen which extends from the base wall and whose effective pervious area is arranged to be increased by an imperforate screen as the imperforate screen is withdrawn from the base wall, applying positive pressure to the dispersion being introduced into the container to compact an accumulating mass of the fibres on the base wall while aiding the discharge of the liquid through the filter screen and progressively in-creasing the separation between the imperforate screen and the base wall so as to increase the effective pervious area of the filter screen at a rate substantially equal to the rate at which the accummulating mass of fibres extends from the base wall, and removing the mass of fibres after they have accummulated to a desired amount either alone or with the side wall moulded into the surface of the mass.

2. A method according to claim 1, in which the base wall is a liquid pervious base wall through which a portion of the liquid is discharged simultaneously with the discharge of the remaining portion through the filter screen.

3. A method according to claim 1 or 2, in which a separate substantially rigid perforated filter support sheet is mounted in the container in close contact with at least a portion of the filter screen so that the liquid drains through the support sheet and through the filter screen while some of the fibres fill the perforations in the support sheet, whereby the support sheet is removed as a unit with the accummulated mass of fibres from within the filter screen.

4. A method according to claim 1, in which a cylin-drical filter element is formed by introducing the dispersion into a cylindrical container comprising a cylindrical filter screen and including a central core, the imperforate screen being a cylindrical screen surrounding the container, and the positive pressure being applied at one end of the container so that as the imperforate screen is separated from the base wall the fibres are layered at substantially constant density perpendicularly to the axis of the container with a majority of the fibres disposed substantially parallel to one another in a substantially circum-ferential direction about said axis.

5. A method according to claim 4, in which, after the accumulation of a desired tubular mass of fibres between the cylindrical container and the core and prior to the removal of the mass from the container the core is withdrawn and a further core inserted, said further core having a smaller cross-section than said first-mentioned core so as to leave a gap between said further core and said tubular mass of fibres and a further dispersion is introduced into said gap and subjected to positive pressure whereby the liquid in said further disper-sion drains through said -tubular mass and said filter screen while the cylindrical screen is progressively separated from the base wall thereby compacting a cylindrical fibrous layer inside the previously formed tubular mass of fibres to be removed from the container as a unit with the previously formed tubular mass.

6. A method according to claims 1, 4 or 5, in which the mass of fibres is substantially dried before removal from the container.

7. A method according to claim 1, in which the fibres in the mass thereof are bonded together by a synthetic resin after removal from the container.

8. A method according to claim 7, in which the syn-thetic resin is a silicone resin.

9. A method according to claims 1, 4 or 5, in which the dispersion is formed by mixing borosilicate microfibres with water and adjusting the pH value of the dispersion to a value between 2.8 and 3.5.

10. A method according to claims 1, 4 or 5, in which the dispersion comprises fibres mixed with water in the ratio by weight of 0.01% to 0.5% fibres to water.

11. A method according to claim 1, in which the fibres are borosilicate microfibres.

12. A method according to claim 1, in which the said positive pressure is applied by gas pressure.

13. A method according to claim 12, in which the dispersion of fibres is formed in a blending tank by means of an agitator, the dispersion is pumped into a pressure vessel and the dispersion is forced by compressed air from the pressure vessel to apply it against the filter screen, the supply of compressed air being maintained to dry the mass of fibres after the draining of liquid therefrom, and the drained water being collected for return to the blending tank.

14. A self-supporting fibrous filter element com-prising a tubular wall consisting of a mass of fibres compacted substantially constant density throughout the length and thickness of the tubular wall with a majority of the fibres dis-posed substantially parallel to one another in a circumferential direction about the central axis of the tubular wall and layered at substantially constant density substantially perpendicularly to the axis, and a synthetic resin bonding the fibres together.

15. A filter element according to claim 14, including a tubular sheet of apertured material providing additional sup-port for the bonded mass of fibres, one face of the tubular wall being jointed to the sheet with some of the fibres pene-trating through the apertures in the sheet and presenting sur-faces substantially flush with the surface of the sheet remote from the wall.

16. A filter element according to claim 15, in which the tubular sheet is at the inner face of the tubular wall.

17. A filter element according to claim 15, in which the tubular sheet is at the outer face of -the tubular wall.

18. A filter element according to claim 15, 16, or 17, in which the tubular sheet is a substantially rigid open pore sheet.

19. A filter element according to claim 14, in which a further tubular wall of a length equal to that of the first-mentioned tubular wall is disposed adjacent the inside surface of the first-mentioned tubular wall, the further tubular wall consisting of a second mass of fibres different from the mass of fibres in the first-mentioned tubular wall and a synthetic resin bonding the fibres in the second mass together, the fibres in the second mass being compacted and disposed in a manner similar to the fibres in the first-mentioned mass.

20. A filter element according to claim 14, includ-ing end caps at least partially closing the ends of the tubular wall, each end cap and the associated end of the tubular wall being formed with interfitting annular conical surfaces provid-ing a taper seal between the end cap and the associated end of said tubular wall.

21. A filter element according to claim 14, 15 or 20, in which the fibres comprise borosilicate glass fibres.